In crown and bridge prosthodontics, a wide diversification of retainers and pontics can be used in various combinations for constructing a bridge. A ceramic to metal restoration uses a framework of metal as reinforcement for the crown and bridge upon which is applied a fired on coating of a ceramic material such as porcelain. The framework of metal may either be cast or formed from prefabricated units of preformed copings and pontics. In accordance with present practice, a framework may be altered by soldering but otherwise cannot be modified or reinforced without involving investment and casting operations. Present practice is limited because of the unavailability of commercial materials with which to build up or extend the framework. To reinforce a framework without investment and casting requires adding material to the framework which upon heat treatment will become an integral part of the framework. The material must be capable of being molded into a desired shape and must be self-supporting in the molded configuration as well as capable of retaining the shape in which it is molded under heat treatment. To be able to shape the material into a desired configuration, the material should be relatively soft and workable. Under heat treatment, the material should solidify into a rigid mass of metal without losing the shape in which it was molded prior to heat treatment. The material should fuse to the metal framework and should have a hardness characteristic of at least equal but preferably greater than the hardness of the material before heat treatment.
Such a material could be used, for example, to build up a cervical shoulder around a retaining member at the gingival margin without the need for investment or casting. For example, a finishing shoulder can be formed around a prefabricated metal coping which was preformed without a shoulder margin. The finishing shoulder can be molded into any shape by the dental technician. Likewise, the material can be used to build metal cusps upon a metal coping before ceramic porcelain is added to provide buccal and/or lingual cusp reinforcement. The material may also be used to strengthen joints at predetermined locations in the framework or for general bridgework repair. The latter is, at present, relatively impossible. Heretofore, the dentist and dental technician were essentially limited to use of cast dental structures and to materials useful as solders or fluxes. Neither the conventional solder nor flux is capable of being molded into a self-supporting configuration nor is either material capable of retaining a shape under heat treatment. Soldering alloys are, in fact, designed to melt and flow freely under the heat of a soldering flame and function to join metals by fusion. A flux is a non-oxidizing agent.
In Applicants parent patent application U.S. Ser. No. 723,063, a dental material composition is disclosed for reinforcing a metal framework incorporating a high fusing temperature precious metal component and a low fusing temperature precious metal component. The high fusing component is in a proportion by volume of from 1-15% of the total composition in the dental material. The material is heat treated at a temperature below the melting temperature of the high fusing temperature metal component to cause the low fusing temperature component to substantially melt and flow around the high fusing temperature component. The dental material solidifies into a rigid mass in response to heat treatment while retaining the shape it was given prior to heat treatment. However, during heat treatment, the mass shrinks as it solidifies. The effect of shrinkage although undesirable, is a necessary characteristic of the foregoing dental material to avoid the formation of voids and air bubbles during heat treatment. Nevertheless some shrinkage can be tolerated for most applications. Likewise, extra material may also be added to compensate for shrinkage but adding extra precious metal is an expensive solution and is not very practical.